Temperature Behavior of Intermolecular Interactions in Ionic Liquids

Room-temperature ionic liquids (RTIL) represent a versatile class of chemical systems, comprised only of oppositely charged species, whose bulk properties can be fine-tuned by adjusting molecular structures and, consequently, intermolecular interactions. Understanding the intricate dynamics between the two ionic species can aid the rational design of RTIL as electrolytes facilitating electron- or proton-transfer reactions useful for energy applications. Here, we investigate the temperature dependency of the intermolecular interactions via magnetization transfer between oppositely charged ions by means of 1H-19F heteronuclear Overhauser effect spectroscopy (HOESY) for two ionic liquids, namely [BMIM][BF4] and [BMIM][PF6]. We find that cross-relaxation rates vary significantly over a rather small temperature range, crossing the null point and changing sign. Other NMR observables such as chemical shifts, scalar couplings or auto-relaxation rates are less affected by temperature, making intermolecular Overhauser transfer a sensitive tool to study the dynamics and especially intermolecular interactions in RTILs. Molecular Dynamics (MD) simulations on neat RTILs systems were performed to identify the origin of the observed behavior. Good agreement between simulated and experimental cross-relaxation rates was found at different temperatures. On this basis, we could attribute the observed temperature behavior to the modulation of dynamic properties rather than coordination changes of RTILs. Specifically, it is the change in relative diffusion coefficients that describes the temperature behavior of internuclear interactions, rather than the distance of closest approach. This information underscores the versatility and high thermal stability of ionic liquids.